The present invention relates to centrifugal turbomachinery for use with gas separation systems. In particular, the present invention relates to split-stream centrifugal compressors, vacuum pumps and expanders in which the flow exiting or entering the centrifugal turbomachinery comprises multiple flows at different total pressures.
Gas separation by pressure swing adsorption (PSA) is achieved by coordinated pressure cycling and flow reversals over an adsorbent bed which preferentially adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the mixture. The total pressure is elevated during intervals of flow in a first direction through the adsorbent bed from a first end to a second end of the bed, and is reduced during intervals of flow in the reverse direction. As the cycle is repeated, the less readily adsorbed component is concentrated in the first direction, while the more readily adsorbed component is concentrated in the reverse direction.
Many prior art PSA systems have low energy efficiency, because feed gas for adsorber pressurization as well as for the high pressure production step is provided by a compressor whose delivery pressure is the highest pressure of the cycle. Energy expended in compressing the feed gas used for pressurization is then dissipated in throttling across valves over the instantaneous pressure difference between the adsorber and the high pressure supply. Similarly, in vacuum swing adsorption (VSA) where the lower pressure of the PSA cycle is established by a vacuum pump exhausting gas at that pressure, energy is dissipated in throttling over valves during countercurrent blowdown of adsorbers whose pressure is being reduced. A further energy dissipation occurs in throttling of light reflux gas used for purge, equalization, cocurrent blowdown and product pressurization or backfill steps. The energy dissipation in irreversible throttling becomes more important when such throttling takes place over larger pressure differences between an adsorber and a feed source or an exhaust sink.
Energy efficiency has been improved in more modern VSA air separation systems, by using feed compressors (or blowers) whose delivery pressure follows the instantaneous pressure of art adsorber being pressurized, and by using vacuum pumps whose suction pressure follows the instantaneous pressure of an adsorber undergoing countercurrent blowdown. In effect, the feed compressor rides each adsorber in turn to pressurize it with reduced throttling losses, and likewise the vacuum pump rides each adsorber in turn to achieve countercurrent blowdown with reduced throttling losses. In such systems, each feed compressor can only supply gas to a single adsorber at any time, and each vacuum pump can only exhaust a single adsorber at a time. The working pressure in each such feed compressor or vacuum pump will undergo large variations, stressing the machinery and causing large fluctuations in overall power demand. Further, compression efficiency is compromised by the unsteady operating conditions.
Since centrifugal or axial turbomachinery cannot operate under such unsteady conditions, rotary positive displacement machines are typically used in VSA systems. However, such machines have lower efficiency than modem centrifugal turbomachinery working under steady conditions, particularly for larger plant ratings (e.g. 50 tons per day oxygen VSA systems). Further, scale up above single train plant capacities of about 80 tons per day oxygen is inhibited by the maximum capacity ratings of single rotary machines.
Other modem VSA air separation systems have used multiple individual impellers to increase the enthalpy of the individual streams. However, these latter systems increase system complexity and capital cost. Furthermore, machine efficiency is reduced since the flow rates are smaller for each machine.
Accordingly, there is a need for centrifugal turbomachinery which can be used in PSA and VSA gas separation processes for maintaining steady conditions of gas flow and pressure, while minimising energy dissipation in irreversible throttling.
According to the invention, there is provided a gas separation system which addresses the deficiencies of the prior art gas separation systems.
As herein mentioned, the term xe2x80x9ccentrifugal turbomachineryxe2x80x9d includes centrifugal compressors, vacuum pumps and expanders.
The gas separation system, according to the invention, separates a feed gas mixture into a first gas component and a second gas component and comprises a stator, and a rotor rotatably coupled to the stator. The stator includes a first stator valve face, a second stator valve surface, and a plurality of function compartments opening into the stator valve surfaces. The rotor includes a first rotor valve surface in communication with the first stator valve surface, and a second rotor valve surface in communication with the second stator valve surface. The rotor also includes a plurality of rotor flow paths for receiving gas adsorbent material therein for preferentially adsorbing the first gas component in response to increasing pressure in the rotor flow paths in comparison to the second gas component.
Each rotor flow path includes a pair of opposite ends opening into the rotor valve faces for communication with the function compartments. The gas separation system also comprises centrifugal turbomachinery coupled to a portion of the function compartments. The centrifugal turbomachinery includes an impeller which has a plurality of impeller flow paths for exposing each rotor flow path to a plurality of discrete pressures as the rotor rotates for separating the first gas component from the second gas component.
In a first embodiment of the invention, the centrifugal turbomachinery comprises a split stream centrifugal compressor for delivering the feed gas mixture to the first stator valve surface at a plurality of different feed gas pressure levels. The centrifugal compressor comprises a gas inlet for receiving the feed gas mixture, a plurality of blades extending radially outwards from the axis of rotation of the impeller, and a channel disposed within the impeller in communication with the gas inlet and extending between adjacent pairs of the blades. The blades include a plurality of steps positioned at differing radial distances from the rotational axis and define impeller flow paths for ejecting the feed gas mixture from the channel at a plurality of different angular momentums. The centrifugal compressor also includes a plurality of diffusers in communication with the channel for providing gas flows at a plurality of different pressures. In one variation of the centrifugal compressor, instead of the blades having steps, the blades have respective blade angles which define the impeller flow paths.
In a second embodiment of the invention, the centrifugal turbomachinery comprises a split stream centrifugal vacuum pump for producing a first product gas from gas flows which are enriched in the first gas component and which are received at a plurality of different sub-atmospheric gas pressure levels from the first stator valve surface. In a third embodiment of the invention, the centrifugal turbomachinery comprises a split stream centrifugal expander for producing a first product gas at atmospheric pressure from gas flows which are enriched in the first gas component and which are received at a plurality of different superatmospheric exhaust gas pressure levels from the first stator valve surface. The centrifugal vacuum pump and the centrifugal expander are structurally similar to the centrifugal compressor except that the direction of gas flow through the impeller flow paths is reversed. In two variations, the centrifugal vacuum pump and the centrifugal expander are coupled to the centrifugal compressor for assisting the centrifugal compressor in delivering the feed gas mixture to the first stator valve surface.
In another embodiment of the invention, the centrifugal turbomachinery comprises a double-sided impeller, a plurality of blades extending radially outwards from the impeller, a first gas inlet and a first gas outlet communicating with a first side of the impeller, and a second gas inlet and a second gas outlet communicating with a second side of the impeller. A first channel is disposed within the first side of the impeller for passing gas between the first gas inlet and the first gas outlet, and a second channel is disposed within the second side of the impeller for passing gas between the second gas inlet and the second gas outlet, with the first and second channels each extending between adjacent pairs of the blades. This latter embodiment may be configured as a split stream centrifugal compressor, a split stream centrifugal vacuum pump and a split stream centrifugal expander with the different impeller flow paths being defined either by a stepped impeller or differing blade angles.
In operation, the feed gas is delivered to the rotor flow paths through the first rotor-stator valve surface pair, and the rotor is rotated at a frequency so as to expose the gas mixture in each rotor flow path to cyclical changes in pressure and direction of flow. These cyclical changes cause the more readily adsorbed component of the feed gas to be exhausted as heavy product gas from the first rotor-stator valve surface pair and the less readily adsorbed component to be delivered as light product gas from the second rotor-stator valve surface pair. To enhance gas separation, light reflux exit gas is withdrawn from the second rotor-stator valve surface pair and is returned after pressure letdown to the second rotor-stator valve surface pair.
In order for the flowing gas streams entering or exiting the centrifugal turbomachinery at each pressure level to be substantially uniform in pressure and velocity, the feed gas is delivered to the rotor flow paths through a plurality of incremental feed gas pressure levels, and the heavy product gas is exhausted from the rotor flow paths as countercurrent blowdown gas through a plurality of decremental exhaust gas pressure levels. Preferably, the light reflux exit gas is withdrawn from the rotor flow paths through a plurality of decremental light reflux exit pressure levels and returned to the rotor flow paths as light reflux return gas at pressure levels less than the respective light reflux exit pressure level. For thermally boosted energy recovery, heat exchangers may also be provided to reject heat of compression and to heat the countercurrent blowdown and the light reflux gas streams about to be expanded.
Preferably the rotor also has a large number of adsorbers such that several adsorbers are exposed to each pressure level at any given moment. During pressurization and blowdown steps, the pressures of the adsorbers passing through each of these steps converge to the nominal pressure level of each step by a throttling pressure equalization from the pressure level of the previous step experienced by the adsorbers. Flow is provided to the adsorbers in a pressurization step or withdrawn in a blowdown step by the centrifugal turbomachinery at the nominal pressure level of that step. Hence flow and pressure pulsations seen by the centrifugal turbomachinery at each intermediate pressure level are minimal by averaging from the several adsorbers passing through the step, although each adsorber undergoes large cyclic changes of pressure and flow.